Generally, flat panel displays may be classified into emissive displays and non-emissive displays.
The emissive displays include a plasma display panel (PDP) display and a field emission display (FED), and the non-emissive displays include a liquid crystal display (LCD).
While the LCD, a non-emissive display, has light weight and low power consumption, it cannot create an image by self-emission, but only by light entering from outside. Thus, the image created by the LCD cannot be observed in a dark place. To solve this problem, a back light unit is equipped at a back surface of the LCD.
Conventionally, as a back light unit, a cold cathode fluorescent lamp (CCFL), which is a linear light source, and a light emitting diode (LED), which is a spot light source, are mainly used.
However, such back light units are complicated and expensive, and consume a large amount of power according to reflection and transmission of light due to the light source disposed at a side thereof. Particularly, as the LCD becomes larger, uniform brightness becomes difficult to ensure.
For these reasons, in recent time, a field emission back light unit having a flat emission structure has been developed. Such a field emission back light unit consumes less power and exhibits relatively uniform brightness in a wide range of an emission region, compared to the conventional back light unit using a CCFL.
Generally, in a field emission-back light unit, a cathode substrate having a field emitter and an anode substrate having a phosphor are disposed to face and be spaced a specific distance apart from each other, and vacuum-packed. An electron emitted from the field emitter collides with the phosphor of the anode substrate, so that light is emitted by cathodoluminescence of the phosphor.
A structure of the conventional field emission back light unit will now be described with reference to FIG. 1.
FIG. 1 illustrates the field emission back light unit capable of local dimming, which has a metal gate substrate.
Referring to FIG. 1, the conventional field emission back light unit having the metal gate substrate includes a cathode substrate as a lower substrate 110, a plurality of cathode electrodes 112 formed on the lower substrate 110, a field emitter 114 formed on the plurality of cathode electrodes 112, an anode substrate as an upper substrate 120, an anode electrode 122 formed on the upper substrate 120, a phosphor layer 124 formed on the anode electrode 122, a metal gate substrate 132 and spacers 142 and 144.
When a specific amount of current is applied to the cathode electrode 112, the field emitter 114 emits an electron beam, which is radially propagated. As a result, the electron beam emitted from the field emitter 114 reaches a phosphor 124 corresponding to a pixel, and the phosphor 124 emits light.
Meanwhile, for partial control of brightness and a high contrast ratio, an amount of current applied to the plurality of cathode electrodes 112 is controlled. For precise local dimming, the number of cathode electrodes 112 has to be increased, which makes interconnections connected with an external electrode complicated.
Thus, a method for effectively performing local dimming without complicated interconnections connected with an external electrode is needed.